We report the first measured activation parameters for the additions of CCl2 and CCIF to simple alkenes and demonstrate the existence of enthalpic barriers for CCl2 additions to cyclohexene and 1-hexene. With these two alkenes, additions of PhCCl are "dominated" by entropic contributions to Delta G(double dagger) and additions of CCl2 display comparable contributions of Delta H-double dagger and Delta S-double dagger, while CCIF additions feature domminant enthalpic contributions to Delta G(double dagger). Entropic factors, however, control the additions of all three carbenes to the highly reactive alkene, tetramethylethylene. Parallel computational studies are also reported.